1. Field of the Invention
This invention relates to processes for purifying polymers, and more particularly, this invention relates to processes for separating high molecular weight fractions from polymers containing a range of molecular weights.
2. Description of the Prior Art
As polymer technology is extended into more product areas, there is an increasing need for purification of polymeric substances. The nature of the impurities found in polymers depends on the origin of the polymers. In the case of synthetic polymers, the impurities are usually associated with the method of synthesis. In the case of natural polymers, the impurities are associated with the natural source of the polymer and the method of extraction or separation from that source.
In addition to simple removal of nonpolymeric impurities, it is frequently important to separate polymeric substances from relatively low or relatively high molecular weight species of the same polymeric substance. The reasons for this need to separate specific narrow molecular weight fractions having improved physical and chemical properties from a mixture are well known in the art but will become further apparent later in this disclosure.
There are many processes reported which are either expensive to operate or suitable only for laboratory preparation of pure, specific molecular weight fractions of polymeric substances. The most important of these processes include precipitation from solution by addition of a nonsolvent liquid, chromatographic separation, sedimentation techniques, diffusion techniques, and ultrafiltration using porous membranes.
U.S. Pat. No. 2,945,016 relates to a process for separation of polymers comprising adding an open chain hydrocarbon to a solution of normally solid polymer in a cyclic hydrocarbon solvent, thereby causing the formation of a polymer-rich phase and a solvent-rich phase, and recovering at least one of said phases.